Ionizing irradiation polymerization of trioxane with copolymerizable stabilizing comonomers



United States Patent ()fifice Patented Jan. 30, 1968 This application is a continuation-in-part of application Ser. No. 182,126 filed Mar. 23, 1962, now abandoned.

The present invention relates to the production of copolymers of formaldehyde, and more specifically to copolymers of formaldehyde which possess an improved degree of thermal stability and superior physical properties.

It has long been known that valuable high molecular weight polyoxym'ethylene polymers may be obtained by polymerizing an oxymethylene precursor such as trioxane in the presence of suitable polymerization catalyst.

Early polyoxymethylene polymers while exhibiting many desirable physical properties were found to be extremely susceptible to thermal degradation. It was found that when heated, the polyoxymethylene molecular would tend to degrade (or unzip) with the evolution of gaseous formaldehyde. In order to retard the unzipping reaction which occurs when polyoxymethylene is heated, it has generally been the practice to copolymerize polyoxymethylene with a comonomer which acts as a blocking or capping agent. It is reasoned that a blocking or capping agent when bonded to an end of a polymeric polyoxymethylene chain will serve to protect the otherwiservulnerable terminal oxymethylene radical from removal.

While in practice it is found that comonomers do in fact render polyoxymethylene more stable towards unzipping reaction, it is also found that capping comonomers serve the adverse secondary function of blocking polymeric growth. This occurs when the reactive terminal polyoxymethylene radical is capped with a comonomer radical which, in addition to protecting the terminal oxymethylene radical from removal, also ties up the radical and terminates chain growth. Therefore, it is frequently found that stabilized copolymers of polyoxymethylene possess lower molecular weights and melting points than the corresponding homopolymers.

It is therefore an object of the present invention to provide a method for preparing polyoxymethylene copolymers which possess thermal stability and desirable physical properties.

It is another object to provide a high molecular weight polyoxymethylene copolymer which is stable at elevated temperatures and which possess a molecular weight within a desirable range.

These and still further objects of the present invention will become readily apparent to one skilled in the art mm the following detailed description and specific examples.

Broadly the present invention contemplates a method for producing polyoxymethylene copolymers which comprises copolymerizing an oxymethylene yielding compound and a stabilizing comonomer in a solid state system which utilizes high energy irradiation as the polymerization initiation force.

More specifically the present invention involves copolymerizing an oxymethylene yielding precursor such as trioxane or other low molecular weight polyoxym'ethylene polymer and a stabilizing amount of a copolymerizable stabilizing comonomer such as a cyclic ether or a substituted vinyl compound, in the solid state wherein high energy irradiation is used as the polymerization initiator.

The process envisioned herein will produce a polyoxymethylene copolymer which possesses polyoxymethylene polymeric chains having a molecular weight substantially the same as a polyoxym'ethylene homopolymer which has been subjected to similar polymerization conditions.

It is found that the stable polyoxyrnethylene copolymer of the present invention may be conveniently produced by either one or two general preparative methods.

The first and perhaps the generally preferred procedure involves the steps of (1) blending from about 0.1%

to about 5% of a comonomer with a mass of trioxane which has been heated to above its melting point, (2) cooling the comonomer-trioxane mixture to below its solidification temperature, (3) irradiating the solidified mixture with from about 0.01 to about 2.0 megarads of high energy radiation and (4) heating the irradiated mixture to a temperature of from about 25 to about 62 C. for a period of from about 0.5 to about 50,h0urs to allow polymer chain growth to occur.

An alternative procedure involves 1) irradiating trioxane in the solid state at room temperature with from about 0.01 to about 2.0 megarads of high energy radiation, (2) aging the irradiated trioxane at a temperature of from about 25 to about 62 C. for a time of 15 minutes to 5 hours to allow chain growth to occur, (3) blending the irradiated and aged polymer with from about 0.1 to about 5.0% of the stabilizing comonomer at a temperature above the melting point of the mass, and (4) aging the comingled mass at a temperature of from about 25 C. to about 62 C. for a period of from about 0.5 to about 5.0 hours to complete the polymeric growth.

Since the stabilizing comonomers do not cocrystallize with the trioxane, polymerization occurs within the trioxane crystal to form a large polyoxymethylene block before copolymerization or capping occurs with the comonomer. This perm-its practically the same degree of chain growth that would be expected in absence of comonomer. Therefore it is found that the molecular weight and melting point of the copolymer is approximately that of the corresponding homopolymer. By using the alternative method mentioned above, wherein some polymerization of the trioxane occurs before contact with the copolyrnerizable comonomer, it is found that a larger homopolymer block is formed, and a correspondingly higher melting point but somewhat less stable product is obtained. This is explained by the fact that a higher degree of polymerization is obtained before the chain stopping comonomer is added, which results in longer polymer chains but less copolyrnerization.

In general, in the above procedures the blending of the comonomer with the trioxane may be carried out at a temperature above the melting point of the trioxane (or partially polymerized trioxane) but below the decomposition temperatures thereof. Generally speaking temperatures in the neighborhood of about C. are preferred in that higher temperatures may cause excessive loss of low boiling monomers or the partial thermal polymerization thereof. 7

When the comonomer-trioxane mixture is cooled to effect the crystallization thereof, temperatures in the neighborhood of room temperature (20-25 C.) are entirely satisfactory. Subsequent to crystallization the mixture may be subjected to irradiation at substantially the same temperatures achieved upon solidification (normally room temperature). The exact dosage of irradiation applied will depend on the degree of polymerization desired. In general, an irradiation dosage of from about 0.01 to about 2.0 megarads may be effectively used.

The aging step subsequent to irradiation wherein polymeric growth occurs is conducted at a temperature above about 25 C. but below the melting point of the polymer which is generally in the range of 25 to 62 C. A temperature range wherein a substantial rate of chain growth occurs is generally somewhat above 25 C. and optimum Having described the essential aspects of the present invention the following specific examples are given to illustrate specific embodiments thereof.

results are generally obtained at about 40 to 62 C. 5 Example I Irradiation which is employed in the instant invention may be conveniently Obtained f a Van de G fi A series of 160 g. samples of trioxane was melted at a electron accelerator, however it should be understood that templatur? of and y Welght) 0f f l high energy particles such as protons, alpha particles and chlomlhydrm, p j y QXlde, dlethyl mtlleater deutrons as well as neutrons may be used in the practice PYTrOhdOne, f y py 2 acrylamlde f added f the present invention Furthermore high energy m". to each. The mixtures were stirred for a few mmu-tes to cles Such as Xqays and gammaqays may be used To Cmate obtain homogeneous blends. The blended mixtures were the activated specie which is responsible for the polymerithen formed into P q of Thickness and lzafion f the herein intended compositions lowed to crystallize at room temperature. The solld In general the stabilizing comonomers which may be 15 P q Were irradiated at temperature to a utilized in the practice of the present invention are those dose of 063 MR 111 a Van de aaff electron acceleratorwhich will react with the terminal oxymethylene radicals The irradiated P q were then Placed in 250 Screw of a polyoxy-methylene chain. In general preferred types p bOtfleS and aged at 55 for 5 of comonomers may be described as cyclic ethers having The Percentage Conversions. and meltlllg P s Of the the formula respective samples are given in Table I below. Furthermore, percentage weight losses are given for two selected times and temperatures which indicate that the thermal R (i (CH2)u stability of the respective copolymers are superior to that of the corresponding homopolymer.

TABLE I 105 C. Testing, 150 0. Testing, Percent RSV in 'y- Percent Wt. Loss Percent Wt. Loss Comonomer Conversion butyrolaetone M. P., C.

96 hrs. 336 hrs. 24 hrs. 72 hrs.

42. 1 1. 09 181182 16. 910. 7 46. 411. 2 64. 6310.4 81. 4i[). 8 15.2 0. 73 181-183 8. :0.1 13. 111:0.9 36. 8510.2 49 :|:5 Propylene OXlde 37.6 1.22 183-186 14. 75:13.6 43 i8 97 i2 98 i2 Diethyl Maleate 21.9 0. 66 180-183 14.4;l:0.2 26 45:04 58 i3 73 i5 vinylpyrrolidonefl 16. 3 0. 86 182-184 7. 610. 4 10. 6:0. 5 24. 211. 2 32. 65:1. 9 Vinylpyridine 10. s 0. 73 183-186 7. 911:0. 2 11. 4i0. 0 29. 75:2. 43. 512. 7 Acrylamide 8. 1 0. 3 180- 82 18. 0:1:0. 6 24. 210. 4 42. 85:1. 0 59. 35:0. 4

wherein R may be hydrogen, alkyl having 1 to 16 car- The above copolymers when observed under the microbon atoms, lower hal-oalkyl having '1 to 5 carbon atoms, scope are extremely fiberous in nature. A single crystallower alkoxy lower alkyl, lower alkoxyphenyl, and cyano line fiber was isolated from the samples containing colower alkyl and n is an integer having a value of O to 1; polymerized Viny-l-pyrrolidone and vinylpyridine. These unsaturated esters having the formulae crystalline fibers were then examined by a standard X-ray diffraction pattern technique which is generally disclosed H in M. J. Buerger, X-ray Crystallograph (1942). In this 6 R2 example the specific X-ray radiation used was the K alpha (0112) doublet of copper. A rotating crystal X-ray camera having a 5.73 centimeter radius as manufactured 'by Charles II Supper Company of Newton Centre, Massachusetts was used to record the diffraction pattern. The polymer mate- (0H2)u rial was mounted in a manner that positioned the polymer 0 fiber axis coaxial with the rotation axis of the camera, and a diffraction pattern was prepared. The distances 2 from an und-ifi'racted spot on the photograph to each of the wherein R is lower alkyl having 1 to 5 carbon atoms, R f' Spots Weft? r is hydrogen or methyl, and n is an integer having a Value Uslng the relatlon T =1 ambda/$1I1 tan I a of 0 to vinyl compounds having the formula wave length of the radiation (1.5 33 A and R 1s the radius of the camera (5.73 cm.) 1dent1ty distances. for R4-CR5:CH2 T were calculated from the measured values of Y. The wherein R may be phenyl, amido, pyridyl, pyrrolidinyl, Values ain d for T for each example are given below. alkyl having 1 to 10 carbon atoms, and cyano, and R Comonomer: Idwtity period (A) 15 hydrogen methyl Vinylpyrrolidone '14 2 10,5 The copolymers produced by the present 1nvent1on possess a novel fibrous-crystalline structure. when these 55 463+?) 10 fibers are examined by a standard X-ray crystallographic :6 technique, they are found to possess an identity period of vinylpyridine 5 73 14 A. along the fiber ax1s. That 15, within experimental va- 7.1+0 2 ri-ation the identity distances found for the present copolymers are equal to or submultiples of an identity period 70 3 8 s of 14 A. On the other hand, it is generally known that copolymers of the same general chemical composition Examination of the above data indicates that within produced by conventional prior art methods, i.e. chemiexperimental variations the identity distance is found cal induced polymerization methods, possess an identity or equal to or s-ubmultiples of an identity period of 14 A. period of 17 A. along the fiber axis. along the polymeric fiber axis. Polyoxymethylene prepared by conventional chemical polymerization techniques possesses an identity period of 17 A. Therefore, it is seen that the present polymers possess a novel crystal-line structure which is not supplicated by the prior art chemical polymerization means.

5 wherein R is lower alkyl; R and R are selected from the group consisting of hydrogen and methyl, R is se- Example H lected from the group consisting of phenyl, amido, pyr- Several 160 g. samples of trioxane were melted and idyl, pyrrolidinyl, alkyl, and cyano, and m is an integer formed into solid plaques having a thickness of 0.5 cm. having a value of -10, in the solid state wherein high The plaques were then irradiated to a dose of 0.3 megarad [0 energy ionizing irradiation is used as the polymerization at room temperature and then placed in 250 cc. screw initiator. cap bottles. The irradiated plaques were aged for 0.5 2. The method of claim 1 wherein the monomer is hours at 55 C. and subsequently melted at 75 C. and diethyl maleate. blended with 2% by weight of the comonomers disclosed 3. The method of claim 1 wherein the monomer is in Example I. The blended samples were then allowed to 15 N-vinylpyridine. solidify and then aged for 4.5 hours at 55 C. The re 4. The method of claim 1 wherein the monomer is spective percentage conversions, melting points, and ther- 2-vinyl-pyridine. mal stabilities of the copolymers obtained by this process 5. The method of claim 1 wherein the monomer is are disclosed in Table II below. acrylamide.

TABLE II 105 0. Testing, 150 0. Testing, Percent RSV 111 Percent Wt. Loss Percent Wt. Loss Comonomer Conversion butyrolactone M. P., C.

96 hrs. 336 hrs. 24 hrs. 72 hrs.

None 32. 9 0. 83 181-182 23. 0:1;4. 5 50i6 85. Oil. 1 97. 9:1:1. 6 Epichlorohydrin 23. 1 0.74 182-183 4. 73:0. 9 2&8 94. 3:2. 0 97.0;110. 5 Propylene Oxide- 21. 1 0. 94 182-185 8. 3:1;1. 7 335115 585:5 67:1:5 Diethyl Maleate 27. 0 0. 92 182-184 8. 43:1. 6 2&5 785:6 87:1:4 N-vinylpyrrolidone 2o. 1 1.00 183-185 5. :0. 8 21:1:5 463:6 57i6 2-vinylpyridine 20. 9 0.91 184-186 4. 06:|:0. 00 6. 810. 2 43. 6:1:1. 2 56. 65:1. 2 Aerylamide 22. 6 1. 09 180-181 4 8:1;0. a=8 80i17 95:1:3

In general it is seen fiom the above examples that no References Cited reduction in melting point occur when copolymers in lieu UNITED STATES PATENTS of homopolymers are formed in accordance with the present invention. Furthermore, it is generally seen when 3115480 12/1963 Steadman 260 67 the comonomer is added prior to irradiation a greater 3,194,510 7/1965 Brown 260*73 stabilizing effect is obtained than when the comonomer OTHER REFERENCES is added after some irradiation has taken place.

l l i Okamura et al.: (V), Radiation Polymerization of 1. A method for producing stable polyoxymethylene TIiOXaIIe, Isotopes and Radiation, VOL 5 copolymers which comprises copolymerizing trioxane and PP- from about 1 to 5.0 percent by weight of a copoly- Okamura et al.: Annual Report Of the Japanese ASSO- merizable monomer selected from the group consisting of ciation for Radiation Research of Polymers. (I) vol. 2, October 10, 1960, p. 223, (11) vol. 3, 1961, pp. 213-219, H1) vol. 3, 1961, pp. 299-310, (IV) vol. 3, 1961, p. 311- 317 (QD 601 AZNS).

MURRAY TILLMAN, Primary Examiner.

SAMUEL H. BLECH, Examiner.

N. F. OBLON, R. B. TURER, Assistant Ex'aminers. 

1. A METHOD FOR PRODUCING STABLE POLYOXYMETHYLENE COPOLYMERS WHICH COMPRISES COPOLYMERIZING TRIOXANE AND FROM ABOUT 0.1 TO 5.0 PERCENT BY WEIGHT OF A COPOLYMERIZABLE MONOMER SELECTED FROM THE GROUP CONSISTING OF 